Seoul, South Korea — Gunfire in the Yellow Sea rattled nerves here Wednesday one week before South Korea faces its greatest security challenge in years: how to protect 20 world leaders and their aides from protests here and the ever-present danger of North Korea provoking an incident to distract attention.

The leaders, including President Obama, arrive here Nov. 10 for two days of meetings of the entire group as well as bilateral sessions intended to resolve such major questions as currency imbalances and the rich-poor gap among nations. By the time they leave two days later, they are expected to have agreed on an “action plan,” however vaguely stated.

“I do not envision North Korea engaged in any inappropriate activities,” Mr. Lee said at a press conference to which foreign correspondents were invited. But just in case, he added, “We’re prepared for any contingency.”

Expect the unexpected

Unlikely though it might appear for North Korea to provoke an incident while President Hu Jintao of China, the source of most of North Korea’s aid, is here, Koreans have come to expect the unexpected.

The sinking of the South Korean Navy vessel the Cheonan in March, with a loss of 46 lives, “reminded many of us the Chinese are thinking in the frame of the 1960s,” said a senior South Korean diplomat. “We no longer live in the world of cold war.”

That comment was a reminder of China’s refusal to endorse the results of a South Korean investigation. The inquiry found North Korea responsible for firing the torpedo that sank the Cheonan in the same waters in which Korea’s Defense Ministry says a patrol boat had fired 10 shots to get a North Korean fishing boat to turn around after two hours on the South Korean side of the “northern limit line.” The North refuses to recognize the line – the reason for several bloody battles in recent years.

South Korean forces were already on alert on their side of the 160-mile-long demilitarized zone that divides the Korean peninsula after a shootout Friday that defense officials say began with North Koreans firing two shots at a South Korean guard post. No injuries were reported after the South Koreans fired three shots back, but a senior official said it was still “not clear” if the North Korean side had deliberately provoked the incident.

Thousands of miles away, yet another incident aroused concerns here – the explosion in Yemen of a pipeline in a facility operated by the Korea National Oil Corp. Whether it was intentional or accidental is not clear. There were no injuries, but the incident prompted much speculation about any relationship to the G20 summit.

Again, President Lee sought to allay apprehensions. “What happened in Yemen will not have any effect on the Seoul summit,” he said. “We are very confident of the safety and success of the summit. I am not concerned about terrorist attack.”

One big reason for such confidence is the mobilization of thousands of policemen around the vast Convention and Exhibition Center where the leaders will meet.

“As the event draws nearer, step by step police forces will be on high alert,” said Cho Hyun-oh, commissioner general of the Korean National Police Agency. “50,000 police officers will be mobilized – the largest number ever.”

Just to be on the safe side, immigration officials have also turned back several hundred foreigners suspected of wanting to enter the country to spur on demonstrations. Protests are expected, but the national police force is under orders to keep them well outside a two-mile-wide security zone surrounding the summit.